1. Field of the Invention
The present invention relates to a motor drive apparatus, and particularly to a motor drive apparatus having an oscillation-reducing control function for output torque.
2. Description of the Background Art
Hybrid vehicles and electric vehicles have recently been of great interest as environment-friendly motor vehicles. A hybrid vehicle has, as its motive power sources, a DC (direct current) power supply, an inverter and a motor driven by the inverter in addition to a conventional engine. More specifically, the engine is driven to secure the motive power source and a DC voltage from the DC power supply is converted by the inverter into an AC (alternating current) voltage to be used for rotating the motor and thereby securing the motive power source as well.
An electric vehicle refers to a motor vehicle that has, as its motive power sources, a DC power supply, an inverter and a motor driven by the inverter.
A motor drive apparatus that is mounted on such a hybrid vehicle or electric vehicle employs an oscillation-reducing control technique that matches an output torque of the motor with a torque command value with high precision to reduce vibrations of the vehicle caused by an error in torque control.
FIG. 19 is a schematic block diagram of an electric-current control apparatus for an AC motor disclosed for example in Japanese Patent Laying-Open No. 09-238492. The current control apparatus shown here employs a so-called vector control technique using a γ-δ coordinate system with which the voltage and current of the motor stator and rotor can be represented by straight lines.
Referring to FIG. 19, an induction motor 102 is driven by three-phase AC current from an inverter 111.
A vector control command value calculator 101 receives, as an input, torque command value T* provided from an external component to calculate and output slip angular velocity command value ωse*, exciting current command value iγs* and torque current command value iδs*.
Slip angular velocity command value ωse* that is output from vector control command value calculator 101 is input to an integrator 115. Exciting current command value iγs* and torque current command value iδs* are input to a current controller 109.
Integrator 115 calculates the integral of slip angular velocity command value ωse* to determine slip angular phase θse and outputs this slip angular phase θse to a power supply angular phase calculator 114 comprised of an adder. A rotational position detector 113 determines rotational angular position θre of the rotor of induction motor 102 based on a signal from an encoder 103 and outputs the determined position to power supply angular phase calculator 114.
Power supply angular phase calculator 114 adds rotational angular position θre to slip angular phase θse to calculate power supply angular phase θ.
A u-phase current sensor 106 detects and outputs u-phase current iu of the stator of induction motor 102 and a v-phase current sensor 107 detects and outputs v-phase current iv of the stator. From these u-phase current iu, v-phase current iv and power supply angular phase θ, a three-to-two phase converter 108 calculates and outputs exciting current iγs and torque current iδs.
From exciting current command value irs* and exciting current iγs as well as torque current command value iδs* and torque current iδs, respectively, current controller 109 calculates and outputs excitation component voltage command value vγs* and torque component voltage command value vδs*.
A PWM (Pulse Width Modulation) generator 110 uses power supply angular phase θ to perform two-to-three phase conversion on excitation component voltage command value vγs* and torque component voltage command value vδs* into a three-phase voltage command value and outputs a three-phase PWM signal to inverter 111. In response to the three-phase PWM signal, inverter 111 supplies to induction motor 102 three-phase AC current (iu, iv, iw).
Regarding the configuration discussed above, rotational angular position θre, which is a component of power supply angular phase θ used for the two-to-three phase conversion by PWM generator 110, can be obtained as an updated and accurate value all the time by encoder 103. Therefore, as compared with the case where the power supply angular phase is determined from the rotational angular velocity that requires a predetermined time for measurement and that is large in measurement error when the velocity changes, the motor output torque can be controlled accurately so that the torque is set to a command value. Accordingly, the body longitudinal acceleration due to an error in torque control that causes transient vibrations in the longitudinal direction of the vehicle's body can be reduced.
Further, the vector control is implemented by digital current control. In sampling the rotational angular position and the actual three-phase current, the sum of the sampling value of the rotational angular position and the slip angular phase is calculated to determine a first power supply angular phase. The sum of the rotational angular velocity and the slip angular velocity is calculated to determine the power supply angular velocity. The power supply angular velocity is used to make compensation for the first power supply angular phase and thereby determine a second power supply angular phase that is used to generate the three-phase PWM signal. Thus, variations in control in a transient state can be reduced.
The three-phase PWM signal generated by PWM generator 110 shown in FIG. 19 is a switching signal that is obtained by comparing excitation component voltage command value vγs* and torque component voltage command value vδs* with a triangular-wave carrier signal. This switching signal can be used to cause elements of inverter 111 to be on/off and thereby obtain an AC output voltage having its average proportional to the amplitude of the voltage command value.
In the PWM control system, in order to cause the elements to be on/off all the time in each cycle of the triangular-wave signal, it is necessary that the amplitude of the voltage command value is smaller than the amplitude of the triangular-wave signal. A resultant problem is that the voltage utilization factor is limited and accordingly a sufficiently high power output cannot be obtained.
As an example of the control system having a higher voltage utilization factor than the PWM control system, a control system that uses a rectangular-wave voltage (rectangular-wave control system) or overmodulation control system is known. The rectangular-wave control system and overmodulation control system use the voltage to the degree that is close to the limitation and thus such systems can increase the motor power output as compared with the PWM control system.
However, the rectangular-wave control system and overmodulation control system are relatively lower in control response than the PWM control system. A resultant problem is that, when a sudden change occurs in torque command value or motor revolution number, an instantaneous drop of the battery is caused for example and accordingly a desired torque cannot be obtained.
In this respect, the PWM control system is advantageous since it has high control response so that torque can be output stably even when a sudden change occurs in load.
Then, with the purpose of increasing the voltage utilization factor for the entire control and controlling the motor stably in the state of transient change where the load suddenly changes, a motor control apparatus that can selectively change the motor control mode between the PWM control and rectangular-wave control is disclosed.
Specifically, Japanese Patent Laying-Open No. 2000-358393 discloses a motor control apparatus that performs control by means of a PWM waveform voltage until the absolute value of a voltage command value of each phase of an AC motor exceeds A/2 (a value equivalent to a battery voltage) and, when the absolute value of the voltage command value becomes equal to or larger than A/2, the control is performed by means of rectangular-wave voltage. Further, when any of respective voltage command values of the phases exceeds the maximum voltage value that can be generated by the inverter, the torque command value is reduced and the voltage command value is calculated again. Furthermore, there is another feature that an ECU for vehicle control is informed of the reduced torque command value.
It is supposed here that, to the motor drive apparatus having such a motor control mode switching function as described above, the above-described oscillation-reducing control is applied.
The oscillation-reducing control controls, as described above, the motor output torque accurately so that the torque matches a command value and reduces variations in control in a transient state. Therefore, as a motor control mode, the PWM control superior in control response shown in FIG. 13 is employed.
It is further supposed here that, according to a voltage command value, the motor control mode is changed from the PWM control to the rectangular-wave control. Under the rectangular-wave control, it is difficult to continue high-precision oscillation-reducing control due to the low control response. Therefore, at the timing when the PWM control is changed to the rectangular-wave control, the motor output torque has its waveform that is not continuous, namely that has a stepped portion. Occurrence of the stepped portion results in vibrations of the vehicle, which discomforts the driver.